Wrist Biomechanics and Mechanism of Wrist Injuries

The wrist joint is a diarthrodial joint composed of two rows of carpal bones. The proximal row articulates with the distal articular surface of the radius, which consists of the scaphoid fossa and the lunate, fossa.

The distal surface of the scaphoid, lunate, and triquetrum articulate with the distal carpal row, comprised of the trapezium, trapezoid, capitates, and hamate. The proximal articulation of the wrist is the radiocarpal joint, the distal articulation is the midcarpal joint.

Six major articulations are

Radiocarpal

Midcarpal

Pisotriquetral

Trapeziometacarpal

Common carpometacarpal

Distal radioulnar joints. Each has multiple subarticulations.

The intrinsic ligaments hold the bones of the proximal and distal carpal rows together.

Each of the carpal bones is unique in its mechanics and possesses a unique center of rotation. The overall motion in the wrist is, essence, the sum of the carpal bones moving on each other, influenced by their articulations, ligamentous attachments, and indirect actions of adjacent tendons.

An understanding of the normal biomechanics helps in to assess the mechanisms of wrist injuries better and determine the appropriate treatment.

Normal Carpal Biomechanics

Wrist motion occurs at radiocarpal and intercarpal or midcarpal joints.

There are three axes of motion

Flexion-extension

Radial-ulnar deviation

Prono-supination

Different movements of wrist are

Flexion is about 65-75 degrees. 40% of the movement is at radiocarpal joint and 60% is at midcarpaljoint.

Extension is also about same degrees and the amount of movement contribution differs. 66% is radiocarpal and 33% is midcarpal

Normal radial deviation is about 15-25 degrees and 90% movement is midcarpal

Ulnar deviation is about 35-50 degrees and is equally divided in radiocarpal and midcarpal joints.

The carpus is biaxial in nature and maintains a constant carpal height ratio during radial and ulnar deviation.

[This ratio is the distance from the base of the third metacarpal to the distal subchondral plate of the radius, divided by the length of the third metacarpal. This normal carpal height ratio is 0.54+_ 0.03.]

The proximal carpal row is an intercalated segment with no tendon attachments. All the tendons that influence wrist motion insert distally, either

Base of the metacarpals

Flexor carpi radialis

Extensor carpi radialis longus and brevis

Extensor carpi ulnaris

Onto the pisiform

Flexor carpi ulnaris

Thus, wrist motion in any plane must be initiated at the distal carpal row. Motion in the proximal carpal row begins only when the extrinsic ligaments crossing the midcarpal joint become taut and the force exerted on the proximal carpal row becomes greater than the frictional forces of the intervening articular segments and the resistance of the antagonistic muscular forces.

Inspite of some motion between the individual bones of the distal carpal row, these bones function as single unit with the index and middle metacarpals.

Wrist motion produces multiplanar motion of the distal carpal row because of gerometry.

With ulnar deviation, the opposite occurs. The distal carpal row inclines ulnarly and flexes and pronates, whereas the proximal carpal carpal bones extend and translate radially. These complex motions are required to maintain the carpal congruency and spatial consistency in all wrist positions.

The scaphoid, because of its position, has potential energy for flexion, while the triquetrum, because of its articulation with the hamate, has potential energy for extension. This potential energy created in the proximal carpal row is facilitated by scapholunate interosseous ligament, which stabilizes the scaphoid and the lunate, and the lunotriquetral interosseous ligament, which stabilizes the lunate and the triquetrum.

The result is a dynamic, balanced lunate within the proximal carpal row. During radial deviation, the proximal carpal row translates toward the ulna, while the distal carpal row inclines toward the radius. Additionally, the scaphoid must flex to avoid impinging on the radial styloid, and the entire proximal row is pulled into flexion as long as there is integrity of the scapholunate interosseous igament.

In ulnar deviation of the carpus, the triquetrum is forced into extension by its helicoid articulation with the hamate. This pushes the remainder of the proximal carpal row into extension by virtue of the force transmission through the LTIL. in contrast, the distal carpal row has negligible intracarpal motion, being generally bound together by very short, stout ligaments with broad insertions.34

Ligaments, in general, are composed of 90% type I collagen and 10% type III collagen and function primarily as viscoelastic structures

The relatively greater strength and viscoelastic nature of the interosseous ligaments reflect their functional characteristics and anatomy. The complex, intercalated movements of the proximal carpal row are facilitated by the interosseous ligaments, which are strong and particularly accommodating to shear stress. The extrinsic wrist ligaments, by contrast, are much weaker and stiffer because there is little need for these structures to have dynamic capabilities.

In spite of knowing the movements, the principle or theory of carpal motion is not fully elucidated.

Following theories have been floated.

Column theory of carpal kinematics

The column theory was first proposed by Navarro in 1935. It divides the wrist into the following three columns:

Radial column, consisting of the scaphoid, the trapezium, and the trapezoid. Scaphoid is center of motion and it is mobile

Central column, including the lunate and the capitates and participate in flexion-extension motion

Ulnar column or medial column, consisting of the triquetrum and the hamate and the motion is rotation.

Row theory of carpal kinematics [Link Theory]

motion occurs within and between rows Muscle contractions impart rotational moments to the proximal row through the distal row, and carpal motion is governed by a combination of ligamentous and articular constraints.

The scaphoid, lunate, and triquetrum rotate in the same primary direction, albeit to different magnitudes, during any motion of the hand.

Combined column-row theory of carpal kinematics

Some have theorized that an individual’s carpal kinematic behavior can be explained by some combination of the columnar theory with the row theory.

It is found that the amount of scaphoid shortening and ulnar translation of the scaphoid varies in a normal distribution.

Also, if the scaphoid shortens more, it translates less. Some authors attribute it to difference in individual laxity.

In radial-ulnar deviation, the scaphoid of very lax wrists moved preferentially in the sagittal plane (flexion-extension), whereas in the more rigid wrists, the scaphoid moved preferentially in the frontal plane (radioulnar deviation).

Oval-ring theory of carpal kinematics

The oval-ring theory functionally depicts the carpus as a transverse ring formed by proximal and distal rows and joined by two physiologic links, one radial and the other ulnar.

The radial link is the mobile scaphotrapezial joint, and the ulnar link is the rotatory triquetrohamate joint.

Mechanism of Wrist Injuries

Radial-Sided Carpal Instabilities

These occur following injury caused by compressive force across a hyperdorsiflexed wrist.

The spectrum of injuries has a common mechanism

Hyperdorsiflexion of the wrist

Forearm pronation

Ulnar deviation of the hand

Axial loading of the wrist through the radial palm and thenar eminence

Four distinct Patterns of injury are collectively called progressive perilunate instability.

That progression occurs in four stages.

Stage I

Disruption of the scapholunate and and radioscphocapitate ligament

Stage II

The capitates and scaphoid separate from the lunate and the triquetrum.

Stage III

Injury continues ulnarly and separates the triquetrum from the lunate. Carpus is completely separated from the lunate resulting in a perilunate dislocation.

Stage IV

Complete lunate dislocation. The perilunate and the lunatedislocations may occur in a dorsal or palmar direction.

Perilunate dislocations with fractures are called t greater arc injuries and are prefixed with trans. The pure greater arc injury propagates in a transscaphiod, transcapitate, and transtriquetral fashion. Typically, these injuries occur secondary to high-energy trauma, and the position of the wrist at the time of injury will determine the fracture-dislocation pattern

They are thought to result from from high-energy three-dimensional loading of the wrist with axial and torsional forces applied to any combination of hyperextension, hyperflexion, and radial or ulnar deviation.

Ulnar-Sided Wrist Ligament Injuries

Some perilunate wrist injuries initiate on the ulnar side of the wrist.

These are lunotriquetral ligament injuries along a spectrum, beginning with simple ligament tearsand progressing to static ventralintercalated segment instability.

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